Slopes of the regressions of specific enzymes and age were calculated for normal and dietarily restricted animals. The data support the concept that the rate of aging of most biochemical variables is less in dietarily restricted animals. The final results of the experiment subjecting 25 month old mice to various degrees of dietary restrictions are now complete. The mean life span of the animals fed the 24% protein diet ad libitum was 29.2 + 0.4 months. Dietary restriction did not significantly increase the life span of any of the animals. Three month old mice fed a high level of cellulose in the diet (50%) for one month consumed approximately 30% less calories and experienced a 5% loss in body weight while the control animals' body weights increased 16%. Thus in spite of increased intake, dietary restriction may be achieved by feeding cellulose in the diet. Data indicate that offering a diet containing 50% of the RDA of vitamins to old mice aged 17 - 19.5 months decreased their life span expectancy by 47 and 30% respectively. Thus, at least in mice, longevity may be increased by ensuring the ingestion of the RDA daily.